For a number of years, especially since the advent of automated clinical analyses, there has been a need for simple and inexpensive means to separate serum from the other constituents of coagulated blood so that various diagnostic tests may be performed on the serum without danger that particulates in the fluid may cause malfunctioning and possible breakdown of the complex and costly automated analysis equipment, or produce incorrect and misleading test results, or both. Although the problems have been recognized in the past, the proposed solutions to those problems have all had major shortcomings.
In one type of centrifugally-activated system, a specially-prepared centrifuge tube (which may also be stoppered, air-evacuated, and serve as a blood collection tube) contains a mass of viscous thixotropic gel at its lower (closed) end. The gel has a specific gravity between that of serum (1.03) and the heavier cellular components (1.09). Therefore, when a blood-filled tube is centrifuged the contents will stratify with the gel assuming a position between the serum (or plasma) and the cellular components. See, for example U.S. Pat. No. 3,852,194. While such a system does result in the formation of a barrier between serum (or plasma) and the solid constituents, no filtering action, and no wiping or cleaning of the tube surfaces above the equilibrium position of the gel barrier, take place. Residual fibrin and cells may remain in the serum layer, either clinging to the wall of the tube or floating freely and, in either event, providing a source of interference during subsequent testing of the serum (or plasma) layer.
In an effort to reduce such problems, serum separators have been devised in which the thixotropic gel is introduced from the stoppered upper end of the tube, rather than from the closed lower end, and migrates downwardly during centrifugation to assume its equilibrium position at the serum-clot interface. (See U.S. Pat. Nos. 3,986,962, 4,055,501, 3,957,654, 3,647,070, 4,012,325). Again, however, in such a system the gel performs no significant wiping and filtering actions. In general, such gel systems not only fail to provide a clean separation between the liquid and solid components of blood, but they also have the further disadvantages of being relatively expensive and having only a limited (approximately six months) shelf life.
In another type of centrifugally-activated serum separating device, a piston is fitted into a centrifuge tube, the piston having a specific gravity between that of serum (1.03) and red cells (1.09). The piston has a resilient tube-contacting periphery and is also provided with one or more openings so that during centrifugation it will migrate from the upper end of the tube into an intermediate equilibrium position at the serum-clot interface. See U.S. Pat. Nos. 3,931,018, 3,951,801, 4,001,122. Such piston-type separators are capable of performing effective separating and wiping actions but only if the tubes are dimensioned to close tolerances, thereby necessitating the use of special tubes which, added to the cost of the separator, result in a relatively expensive assembly.
Other types of centrifugally-activated separators, some of which combine gel barriers with piston-like structures, and others which function as separators only when they are subjected to a further treatment (such as heat treatment) after they have assumed their equilibrium positions, are represented by U.S. Pat. Nos. 4,088,582, 3,909,419, 3,926,646, 3,920,557, and 3,919,085. Generally, most of such centrifugally-activated separators, whether utilizing a gel, or an apertured piston, or a combination of both, are automatically interface-seeking because their specific gravity is set or adjusted at about 1.06, at a point between the specific gravities of serum (or plasma) and the heavier non-fluid blood components.